Observing and estimating of intensive triad interaction occurrence in very shallow water

Abstract

In this study, a series of field measurements were carried out to investigate triad interactions of spectral peak in near shore. The water level fluctuations were recorded at 5 stations with depths varying from 0.8 to 5m along a shore-perpendicular transect at sandy coasts of Nowshahr, located in the southern Caspian Sea coast. Two storms with significant wave height of approximately 1.4m were observed during the measurement period.Using bispectral analysis, a new quantitative index is proposed to investigate temporal and spatial intensity of nonlinear interaction between spectral peak and other harmonics. The proposed index was evaluated for time series of water level data and compared with the bicoherence value of self-spectral peak triad interaction (SSPT); b2(fp,fp). Comparing to SSPT, the proposed new index includes all positive and negative triad interactions with spectral peak. The relative depth, kpd, of non-breaking waves varied from 0.25 to 2.00 along the transect during the study period, where kp is the wave-number and d is the water depth. In general, SSPT increased by decreasing kpd; however, the results showed that in two shallow stations the maximum SSPT did not correspond to the lowest values of kpd. A considerable time lag was observed between occurrence of the most intensive triad interactions and termination of wave breaking in post storm condition. Furthermore, the intensive triad interactions happened several hours after the largest Ursell numbers of non-breaking waves.